Volume oscillators—FBARs (thin-film bulk acoustic resonators) or bulk acoustic wave (BAW) resonators that work with acoustic waves—are based on a piezoelectric base body that is on each of the two main surfaces with an electrode. Such a resonator has a resonance frequency fr that depends on the total thickness L0 of the oscillating base body approximately in accordance with the formulafr=v/2L0 Here, v denotes the velocity of longitudinal waves in the piezoelectric base body. Such resonators can be used, for example, to construct HF (high frequency) filters. In addition, several such resonators can be connected in branching circuits into a filter network, a so-called reactance filter.
The required layer thickness L0 for a resonating BAW resonator in the HF range is in the μm and sub-μm range. Thin-layer processes are therefore required for the production of the layers of the base body.
In order to keep the energy of the acoustic waves inside the base body of the resonator, and to provide a sharp resonance frequency for the resonator, two main construction principles are known that make sufficiently high reflection of the acoustic waves possible at boundaries in order to provide an adequate filtering effect with low acoustic or electrical losses.
One possibility for keeping the energy of the acoustic waves within the base body of the resonator comprises arranging the base body over a hollow space, in which case a membrane can also be arranged between the lower electrode and the substrate. This arrangement is also called a bridge-type resonator.
Other BAW resonators of the mirror type use a so-called acoustic mirror. This type includes a number of layer pairs with alternating layers of materials with higher and lower acoustic impedance. Each of the layers has a layer thickness of one quarter lambda (λ/4) so that at each boundary reflected wave portions are superimposed constructively. In principle, in selecting the layer thicknesses, values would also be possible that correspond to odd multiples of quarter lambdas, thus λ/4, 3λ/4, . . . (2n−1)λ/4, with natural numbers n. To optimize resonator characteristics, mirror-layer thickness can deviate slightly form the λ/4, 3λ/4, . . . (2n−1)λ/4 rule. SiO2 is used in particular as a material with lower acoustic impedance. A heavy metal such as tungsten or molybdenum, or aluminum nitride, is used as a material with higher acoustic impedance. The higher the impedance difference between the two materials, the fewer pairs are used for an acoustic mirror. Some acoustic mirrors need at least two λ/4 layer pairs between the lower electrode and the substrate. With each additional layer, however, the effective coupling of the resonator, and thus the bandwidth, is reduced. In contrast to a resonator of the bridge type, the bandwidth of the resonator can be reduced by up to 30% in this case. With such resonators, it is therefore significantly more expensive to construct a band-pass filter with adequate bandwidth.
Another disadvantage of a BAW resonator of the mirror type is in the complexity of the process for depositing and structuring the multilayer structure required for it. Each λ/4 layer increases the complexity, and thus the cost, of the production process. With the number of necessary layers, errors also become more frequent so that a significant scattering of resonance frequencies of the resonators over an entire wafer and, as a result, the average frequency of filters must be taken into account.
Since the bandwidth of the acoustic mirror is reduced as the number of layer pairs for the acoustic mirror increases, in a duplexer, for example, that has two filters with different passage ranges (pass bands), a separate acoustic mirror would be required for each of the two filters. The complexity of production is thereby increased.
Layers with high dielectric constants, such as the metal tungsten and molybdenum in particular, can result in coupling of electric signals to the substrate, which results in undesired speech overlap and an increase in insertion loss.